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​Rainstorms of Molten Iron Showered the Early Earth

The world's most electrifying machine has simulated the heavy metal storms that helped form a young Earth.
​The Sandia Z-Machine, the closest thing to star power on Earth. Image: Randy Montoya

Another reason the early Earth was basically hell: Not only was its surface getting cooked with radiation and pounded with asteroids, it was raining liquid iron.

That's according to experiments run on the Z-machine, a contraption that looks like the stuff of comic book legend but is actually one of the world's most powerful generators. Using this machine, ​researchers at Lawrence Livermore National Laboratory showed how high-energy asteroid impacts during the end stages of Earth's formation sent plumes of iron vapor rising over the planet's surface.

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Then, in the most epic rainstorms the world has ever seen, molten iron showered the planet before sinking down to its final resting place, the core.

One of the key charges of planetary science is to understand how celestial bodies form and evolve. Generally speaking, planets first form by accretion, a process wherein chunks of space debris smash violently into one another at higher and higher speeds, generating immense heat that causes them to liquify and coalesce. As a young planet grows, these accretionary impacts become more powerful, reaching speeds of up to 100,000 miles per hour.

That's basically how things went down on early Earth. But the end stages of planetary formation, involving the highest impact collisions, are poorly understood. For instance, the fate of iron, Earth's most abundant element, is not well-known. We do know that most of our planet's iron eventually sunk to the core, but how and when it got there is open to debate.

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"One major problem is how we model iron during impact events, as it is a major component of planets and its behavior is critical to how we understand planet formation," Richard Kraus the lead author of the new study, which was just published in Nature, said in a statement. "In particular, it is the fraction of that iron that is vaporized on impact that is not well understood."

To understand what happened to iron while the young Earth was getting walloped with rocks, the researchers used Sandia National Laboratory's Z-Machine, the world's most powerful manmade radiation source. This machine generates massive electrical currents and magnetic fields to produce temperatures and pressures found nowhere else on Earth, making it the ideal tool for studying the violent process of planetary accretion.

The researchers simulated impacts occurring in the late-stages of Earth's formation by smashing together iron and aluminum samples at extremely high shock pressures. They discovered that iron will vaporize at significantly lower impact speeds than previously thought. Some four billion years ago, then, high-impact collisions across the world caused iron-rich rocks to go up in smoke.

"This causes a shift in how we think about processes like the formation of Earth's iron core," Kraus said. "Rather than the iron in the colliding objects sinking down directly to the Earth's growing core, the iron is vaporized and spread over the surface within a vapor plume. After cooling, the vapor would have condensed into an iron rain that mixed into the Earth's still-molten mantle.

This process may also explain why the moon, which is thought to have formed around the same time, lacks an iron rich center, despite having been exposed to similar impacts. Because of the moon's weaker gravity, any vaporized iron may simply have fizzled off into space.

Iron, in addition to comprising a large chunk of the Earth, is an absolute requirement for life as we know it: It's found in our proteins, our blood and our DNA. Sure, a molten metal storm today would spell doom for most organisms, but there's something strangely beautiful about the idea of this critical element showering the world just prior to life's emergence.